Raman spectroscopy is a spectroscopic technique that can be used to identify molecules in a sample. The technique relies on Raman (inelastic) scattering of emitted monochromatic light. The emitted light interacts with molecular vibrations, phonons, or other excitations in the sample, which causes the energy of the emitted photons to be shifted up or shifted down. Information about the vibrational modes in the sample can be inferred from the shift in energy. This information can, in turn, be used to identify the molecules in the sample, since vibrational information is specific to the chemical bonds and symmetry of molecules.
Although spontaneous Raman spectroscopy is a powerful molecular detection technique, Raman-scattered signals tend to be very weak. These signals can be enhanced by many orders of magnitude by using specially patterned structures that locally enhance the electric field of the light source and the emitted light. This technique is known as surface-enhanced Raman spectroscopy (SERS). In SERS, sample molecules are adsorbed on rough metal surfaces and/or by nanostructures. For instance, a liquid sample may be deposited onto a silicon or glass surface having a nanostructured noble metal surface.